refactor: cache-aligned summarization for compaction requests

[lloydzhou]

Summary — Cache-Aligned Summarization

Align the summarization agent's prefix (system prompt, tools, cache-control markers) with the main agent so that providers supporting prompt caching reuse the already-cached prefix on compaction requests. Providers that don't support explicit cache-control markers silently ignore them — no behavior change.

将摘要 agent 的前缀（system prompt、tools、cache-control markers）与主 agent 对齐，使支持 prompt caching 的 provider 在压缩请求时复用已缓存的前缀。不支持显式 cache-control 标记的 provider 会静默忽略，行为无变化。

Cost Example (Claude Sonnet 4, $3.00/$0.30/$15.00 per MTok)

A compaction request with 100k input tokens: 5k prefix (system prompt + tools) + 94k old messages + 1k summary instruction.

	Input cost	Output cost	Total
Without caching	100k × $3.00 = $0.300	500 × $15.00 = $0.0075	$0.3075
Cache-Aligned	99k × $0.30 + 1k × $3.00 = $0.0327	500 × $15.00 = $0.0075	$0.0402
Savings			~87%

This reuse strategy is inspired by bash-agent's cache-aware DP compaction.

此复用策略灵感来源于 bash-agent 的缓存感知 DP 压缩算法。

What Changed

• buildSystemPrompt() — shared system prompt builder (base prompt + MCP instructions), extracted from inline code in Run.
• buildAgent() — shared agent factory: calls buildSystemPrompt(), copies tools (with cache control on the last tool), creates fantasy.Agent. Both Run and Summarize now call this instead of constructing agents independently.
• applyCacheControl(messages) — shared cache marking: clears stale provider options, marks the system message boundary and last 2 messages with cache control. Both PrepareStep closures now call this.
• buildSummaryPrompt — summary instructions are now embedded in the user prompt (via templates/summary.md) rather than as a separate system prompt, so the main agent's system prompt prefix is preserved.

变更内容

• buildSystemPrompt() — 共享 system prompt 构建器（基础提示 + MCP 指令），从 Run 内联代码中提取。
• buildAgent() — 共享 agent 工厂：调用 buildSystemPrompt()、复制 tools（最后一个 tool 添加 cache control）、创建 fantasy.Agent。Run 和 Summarize 均通过此方法构建 agent，不再各自独立构造。
• applyCacheControl(messages) — 共享缓存标记：清除过期的 provider options，标记 system message 边界和最后 2 条消息的 cache control。两个 PrepareStep 闭包均调用此方法。
• buildSummaryPrompt — 摘要指令改为嵌入在 user prompt 中（通过 templates/summary.md），而非作为独立的 system prompt，从而保留主 agent 的 system prompt 前缀用于缓存命中。

[charmcli]

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[lloydzhou]

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